Nitride semiconductor element and nitride semiconductor wafer

ABSTRACT

According to one embodiment, a nitride semiconductor element includes a foundation layer, a functional layer and a stacked body. The stacked body is provided between the foundation layer and the functional layer. The stacked body includes a first stacked intermediate layer including a first GaN intermediate layer, a first high Al composition layer of Al x1 Ga 1-x1 N (0&lt;x1≦1) and a first low Al composition layer. A compressive strain is applied to the first low Al composition layer. Unstrained GaN has a first lattice spacing. The Al x1 Ga 1-x1 N (0&lt;x1≦1) when unstrained has a second lattice spacing. The first high Al composition layer has a third lattice spacing. An Al composition ratio of the first low Al composition layer is not more than a ratio of a difference between the first and third lattice spacings to a difference between the first and second lattice spacings.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is based upon and claims the benefit of priority fromthe prior Japanese Patent Application No. 2012-052342, filed on Mar. 8,2012; the entire contents of which are incorporated herein by reference.

FIELD

Embodiments described herein relate generally to a nitride semiconductorelement and a nitride semiconductor wafer.

BACKGROUND

Light emitting diodes (LEDs), which are examples of semiconductor lightemitting elements that use nitride semiconductors, are used in, forexample, display apparatuses, illumination, etc. Electronic devices thatuse nitride semiconductors are utilized in high-speed electronic devicesand power devices.

In the case where such a nitride semiconductor element is formed on asilicon substrate that has excellent suitability for mass production,defects and cracks occur easily due to differences in lattice constantsand coefficients of thermal expansion. In particular, technology toconstruct a high-quality crystal on a substrate of silicon and the likewith few cracks is desirable.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A to FIG. 1D are schematic views showing the configuration of anitride semiconductor element according to the first embodiment;

FIG. 2A to FIG. 2D are graphs showing characteristics of the nitridesemiconductor element;

FIG. 3A to FIG. 3D are schematic views showing the configuration ofanother nitride semiconductor element according to the first embodiment;

FIG. 4 is a graph showing the characteristics of the nitridesemiconductor elements;

FIG. 5A to FIG. 5D are micrographs showing characteristics of thenitride semiconductor elements;

FIG. 6A to FIG. 6D are schematic views showing the results of X-raydiffraction measurements of the nitride semiconductor elements;

FIG. 7 is a graph showing the dislocation density of the nitridesemiconductor;

FIG. 8A to FIG. 8D are schematic views showing configurations of nitridesemiconductor elements;

FIG. 9A to FIG. 9D are schematic views showing the configuration of thenitride semiconductor wafer according to the second embodiment;

FIG. 10A to FIG. 10D are schematic views showing the configuration ofanother nitride semiconductor wafer according to the second embodiment;and

FIG. 11A to FIG. 11J are schematic views showing the configuration ofnitride semiconductor wafers according to the embodiments.

DETAILED DESCRIPTION

According to one embodiment, a nitride semiconductor element includes afoundation layer, a functional layer and a stacked body. The foundationlayer includes an AlN buffer layer. The foundation layer has a majorsurface. The functional layer includes a nitride semiconductor. Thestacked body is provided between the major surface and the functionallayer. The stacked body includes a first stacked intermediate layer. Thefirst stacked intermediate layer includes a first GaN intermediatelayer, a first high Al composition layer of Al_(x1)Ga_(1-x1)N (0<x1≦1)and a first low Al composition layer of Al_(y1)Ga_(1-y1)N (0<y1<1 andy1<x1). The first GaN Intermediate layer is provided on the foundationlayer. The first high Al composition layer is provided on the first GaNintermediate layer. The first low Al composition layer is provided onthe first high Al composition layer. A compressive strain is applied tothe first low Al composition layer. Unstrained GaN has a first latticespacing along a first axis parallel to the major surface. TheAl_(x1)Ga_(1-x1)N (0<x1≦1) when unstrained has a second lattice spacingalong the first axis. The first high Al composition layer has a thirdlattice spacing along the first axis. An Al composition ratio of thefirst low Al composition layer is not more than a ratio of an absolutevalue of a difference between the first lattice spacing and the thirdlattice spacing to an absolute value of a difference between the firstlattice spacing and the second lattice spacing.

According to another embodiment, a nitride semiconductor wafer includesa substrate, a foundation layer and a stacked body. The substrate has amajor surface. The foundation layer is provided on the major surface andincludes an AlN buffer layer. The stacked body is provided on thefoundation layer. The stacked body includes a first stacked intermediatelayer. The first stacked intermediate layer includes a first GaNintermediate layer, a first high Al composition layer ofAl_(x1)Ga_(1-x1)N (0<x1≦1) and a first low Al composition layer ofAl_(y1)Ga_(1-y1)N (0<y1<1 and y1<x1). The first GaN intermediate layeris provided on the foundation layer. The first high Al composition layeris provided on the first GaN intermediate layer. The first low Alcomposition layer is provided on the first high Al composition layer. Acompressive strain is applied to the first low Al composition layer.Unstrained GaN has a first lattice spacing along a first axis parallelto the major surface. The Al_(x1)Ga_(1-x1)N (0<x1≦1) when unstrained hasa second lattice spacing along the first axis. The first high Alcomposition layer has a third lattice spacing along the first axis. AnAl composition ratio of the first low Al composition layer Is not morethan a ratio of an absolute value of a difference between the firstlattice spacing and the third lattice spacing to an absolute value of adifference between the first lattice spacing and the second latticespacing.

Various embodiments will be described hereinafter with reference to theaccompanying drawings.

The drawings are schematic or conceptual; and the relationships betweenthe thicknesses and the widths of portions, the proportions of sizesbetween portions, etc., are not necessarily the same as the actualvalues thereof. Further, the dimensions and/or the proportions may beillustrated differently between the drawings, even for identicalportions.

In the drawings and the specification of the application, componentssimilar to those described in regard to a drawing thereinabove aremarked with like reference numerals, and a detailed description isomitted as appropriate.

First Embodiment

The embodiment relates to a nitride semiconductor element. The nitridesemiconductor element according to the embodiment includes asemiconductor device such as a semiconductor light emitting element, asemiconductor light receiving element, an electronic device, etc. Forexample, the semiconductor light emitting element includes a lightemitting diode (LED), a laser diode (LD), etc. The semiconductor lightreceiving element includes a photodiode (PD), etc. For example, theelectronic device includes a high electron mobility transistor (HEMT), aheterojunction bipolar transistor (HBT), a field effect transistor(FET), a Schottky barrier diode (SBD), etc.

FIG. 1A to FIG. 1D are schematic views illustrating the configuration ofa nitride semiconductor element according to the first embodiment.

FIG. 1A is a schematic cross-sectional view illustrating theconfiguration of the nitride semiconductor element 110 according to theembodiment. FIG. 1B is a graph illustrating the Al composition ratio(C_(Al)) of a stacked body. FIG. 1C is a graph illustrating a growthtemperature GT (a formation temperature) of the stacked body. FIG. 1D Isa graph Illustrating lattice spacing Ld along an a-axis of the stackedbody.

As illustrated in FIG. 1A, the nitride semiconductor element 110according to the embodiment includes a foundation layer 60, a stackedbody 50, and a functional layer 10. The foundation layer 60 has a majorsurface 60 a. The stacked body 50 is provided between the major surface60 a of the foundation layer 60 and the functional layer 10. The stackedbody 50 includes a first stacked intermediate layer 50 a.

Herein, an axis perpendicular to the major surface 60 a is taken as a Zaxis. One axis perpendicular to the Z axis is taken as an X-axisdirection. A direction perpendicular to the Z axis and the X axis istaken as a Y axis. The functional layer is stacked with the stacked body50 along the Z axis. In the specification of the application, “stacking”includes not only the case of being overlaid in contact with each otherbut also the case of being overlaid with another layer insertedtherebetween. Being “provided on” includes not only the case of beingprovided in direct contact but also the case of being provided withanother layer inserted therebetween.

In this example, the nitride semiconductor element 110 further includesa substrate 40. The foundation layer 60 is disposed between thesubstrate 40 and the stacked body 50 (e.g., the first stackedintermediate layer 50 a). The substrate has a major surface 40 a. Themajor surface 40 a of the substrate 40 is parallel to the major surface60 a of the foundation layer 60.

The substrate 40 may include, for example, a silicon substrate. Forexample, the substrate 40 is a Si (111) substrate. However, in theembodiment, the plane orientation of the substrate 40 may not be the(111) plane and may be, for example, the (100) plane or the (11n) planeorientation (n being an integer). For example, it is favorable to use asubstrate 40 of the (110) plane to reduce the lattice mismatch betweenthe silicon substrate and the nitride semiconductor layer.

A substrate including an oxide layer may be used as the substrate 40.For example, an SOI (silicon on insulator) substrate may be used as thesubstrate 40.

The substrate 40 includes a material that has a lattice constantdifferent from the lattice spacing of the functional layer 10 or amaterial that has a coefficient of thermal expansion different from thecoefficient of thermal expansion of the functional layer 10.

For example, sapphire, spinel, GaAs, InP, ZnO, Ge, SiGe, or SIC may beused as the material of the substrate 40.

There are cases where the nitride semiconductor element 110 is used inthe state in which the substrate 40 is removed. There are cases wherethe nitride semiconductor element 110 is used in the state in which thefoundation layer 60 and the stacked body 50 are removed. There are caseswhere the nitride semiconductor element 110 is used in the state inwhich a portion of the functional layer 10 is removed.

One axis parallel to the major surface 60 a is taken as a first axis.The direction of the first axis may be any direction in the X-Y plane.For example, the a-axis of the crystal may be used as the first axis.

In the case where an axis (the Z axis) that Is perpendicular to themajor surface 60 a of the foundation layer 60 is substantially parallelto the c-axis of the crystal of the foundation layer 60, the a-axis ofthe foundation layer 60 is substantially perpendicular to the Z axis.The a-axis may be any direction in the X-Y plane.

In such a case, the a-axis of the crystal included in the stacked body50 is perpendicular to the Z axis. The a-axis of the crystal included Inthe functional layer 10 is perpendicular to the Z axis. The a-axis ofthe foundation layer 60, the a-axis of the stacked body 50, and thea-axis of the functional layer 10 are substantially parallel to eachother.

Hereinbelow, the case will be described where the c-axis of the crystalis substantially parallel to the stacking direction (the Z axis). Inother words, the a-axis is substantially perpendicular to the Z axis andparallel to the X-Y plane. However, as described below, the first axisof the embodiments may be an axis other than the a-axis.

The foundation layer 60 includes an AlN buffer layer 62 and an AlGaNfoundation layer 63.

The AlN buffer layer 62 is disposed between the AlGaN foundation layer63 and the substrate 40. The AlN buffer layer 62 is formed on thesubstrate 40 to contact the substrate 40.

Chemical reactions between AlN and silicon do not occur easily. Meltbacketching and the like that occur due to reactions between silicon andgallium are suppressed by providing the AlN buffer layer 62 thatincludes AlN in contact with the substrate 40. For example, it isfavorable for the thickness of the AlN buffer layer 62 to be not lessthan 20 nm (nanometers) and not more than 400 nm, e.g., about 100 nm.

The AlGaN foundation layer 63 is formed on the AlN buffer layer 62. TheAlGaN foundation layer 63 includes Al, Ga, and N. For example, it isfavorable for the Al composition ratio of the AlGaN foundation layer 63to be not less than 0.1 and not more than 0.9. It is more favorable tobe not less than 0.2 and not more than 0.6, e.g., 0.25. The Alcomposition ratio is the proportion of the number of the Al elementatoms to the number of the group III element atoms. The AlGaN foundationlayer 63 is not limited to being one layer; and multiple layers havingdifferent Al composition ratios may be stacked. In such a case, it isfavorable for the Al composition ratio to gradually decrease in thedirection from the AlN buffer layer 62 toward the functional layer 10.The defects that occur due to lattice mismatch are suppressed bystacking multiple layers having different Al composition ratios.

The suppression effect of the meltback etching can be increased by theAlGaN foundation layer 63. Also, a compressive stress is formed insidethe AlGaN foundation layer 63; and the tensile stress that occurs due tothe difference of the coefficient of thermal expansion between thesubstrate 40 and the nitride semiconductor (e.g., the functional layer10) in the cooling process after the crystal growth is reduced. Thereby,the occurrence of cracks can be suppressed. For example, it is favorablefor the thickness of the AlGaN foundation layer 63 to be not less than100 nm and not more than 500 nm, e.g., about 250 nm.

In the case where multiple nitride semiconductor layers having mutuallydifferent compositions are stacked, the nitride semiconductor layer(e.g., the AlGaN foundation layer 63) that is stacked on top is formedto match the lattice spacing (the length of the lattice) of the nitridesemiconductor layer (e.g., the AlN buffer layer 62) that is formedunderneath. Therefore, the actual lattice spacing of the nitridesemiconductor layer is different from the unstrained lattice spacing(the lattice constant).

In the specification, “lattice constant” refers to the unstrainedlattice spacing of the nitride semiconductor. “Lattice spacing” refersto the length of the actual lattice of the nitride semiconductor layerthat is formed. For example, the lattice constant is a physical propertyconstant. For example, the lattice spacing is the length of the actuallattice of the nitride semiconductor layer included in the nitridesemiconductor element that is formed. For example, the lattice spacingis ascertained from X-ray diffraction measurements.

In this example, the foundation layer 60 further includes a GaNfoundation layer 61. The AlGaN foundation layer 63 is disposed betweenthe GaN foundation layer 61 and the AlN buffer layer 62. In other words,the GaN foundation layer 61 is formed on the AlGaN foundation layer 63.Compressive stress occurs easily in the crystal growth of the stackedbody 50 by providing the GaN foundation layer 61 on the AlGaN foundationlayer 63. Thereby, the occurrence of cracks can be suppressed.

When forming the GaN foundation layer 61 that has a lattice constantthat is greater than the lattice spacing of the AlGaN foundation layer63, the GaN foundation layer 61 is formed to have lattice matching withthe lattice spacing of the AlGaN foundation layer 63; and compressivestress Is formed in the GaN foundation layer 61. As the film thicknessof the GaN foundation layer 61 increases, lattice relaxation occurs inthe GaN foundation layer 61; and the lattice spacing of the GaNfoundation layer 61 approaches the lattice spacing of unstrained GaN. Inthe case where the actual lattice spacing of the GaN foundation layer 61is substantially the same as the lattice spacing of unstrained GaN (thelattice constant of GaN), the compressive stress that would be appliedto the GaN foundation layer 61 does not occur even as the film thicknessis increased; and the tensile stress from the substrate 40 has a greatereffect. By appropriately setting the thickness of the GaN foundationlayer 61, the state can be maintained in which the lattice spacing alongthe first axis (e.g., the a-axis) of the GaN is smaller than the latticespacing along the first axis (e.g., the a-axis) of unstrained GaN (thelattice constant of GaN). For example, it is favorable for the thicknessof the GaN foundation layer 61 to be not less than 100 nm and not morethan 1000 nm, e.g., about 400 nm. The GaN foundation layer 61 may beprovided if necessary and may be omitted in some cases.

The stacked body 50 is formed on the foundation layer 60. The stackedbody 50 includes a GaN intermediate layer 51 (a first GaN intermediatelayer 51 a), a high Al composition layer 52 (a first high Al compositionlayer 52 a), and a low Al composition layer 53 (a first low Alcomposition layer 53 a). The GaN Intermediate layer 51 is providedbetween the low Al composition layer 53 and the foundation layer 60. Thehigh Al composition layer 52 is provided between the low Al compositionlayer 53 and the foundation layer 60. In other words, the low Alcomposition layer 53 is provided on the high Al composition layer 52;and the high Al composition layer 52 is provided on the GaN intermediatelayer 51.

The high Al composition layer 52 (the first high Al composition layer 52a) includes Al_(x1)Ga_(1-x1)N (0<x1≦1). The high Al composition layer 52(the first high Al composition layer 52 a) may include, for example,AlN. The low Al composition layer 53 (the first low Al composition layer53 a) includes Al_(y1)Ga_(1-y1)N (0<y1<1 and y1<x1).

For example, it is favorable for the thickness of the GaN intermediatelayer 51 to be not less than 100 nm and not more than 1000 nm, e.g.,about 300 nm.

For example, it Is favorable for the Al composition ratio of the high Alcomposition layer 52 to be not less than 0.5 and not more than 1.0,e.g., about 1.0. In the case where the Al composition ratio of the highAl composition layer 52 is smaller than 0.5, it is difficult for thehigh Al composition layer 52 to be sufficiently relaxed. For example, itis favorable for the thickness of the high Al composition layer 52 to benot less than nm and not more than 100 nm, e.g., about 12 nm. In thecase where the thickness of the high Al composition layer 52 is thinnerthan 5 nm, it is difficult for the high Al composition layer 52 to besufficiently relaxed. In the case where the thickness of the high Alcomposition layer 52 is thicker than 100 nm, the crystal quality of thehigh Al composition layer 52 degrades easily. For example, the surfaceflatness worsens; and pits occur easily. It is more favorable for thethickness of the high Al composition layer 52 to be not less than 10 nmand not more than 30 nm. The degradation of the crystal quality Issuppressed further in the case where the thickness of the high Alcomposition layer 52 is not more than 30 nm.

The low Al composition layer 53 includes Al, Ga, and N. For example, itis favorable for the Al composition ratio of the low Al compositionlayer 53 to be not less than 0.1 and not more than 0.9, e.g., about 0.5.For example, it is favorable for the thickness of the low Al compositionlayer 53 to be not less than 5 nm and not more than 100 nm, e.g., about25 nm. The Al composition ratio and the thickness of the low Alcomposition layer 53 are described below.

The functional layer 10 is formed on the stacked body 50.

In the case where the nitride semiconductor element 110 is a lightemitting element, for example, the functional layer 10 includes ann-type semiconductor layer 11 formed on the stacked body 50, a lightemitting layer 13 formed on the n-type semiconductor layer 11, and ap-type semiconductor layer 12 formed on the light emitting layer 13. Thelight emitting layer 13 includes multiple barrier layers of GaN, andInGaN (e.g., In_(0.15)Ga_(0.85)N) layers provided between the barrierlayers. The light emitting layer 13 has a MQW (Multiple Quantum Well)structure or a SQW (Single-Quantum Well) structure. For example, it isfavorable for the thickness of the functional layer to be not less than1 micrometer (Lm) and not more than 5 μm, e.g., about 3.5 μm. Thus, thefunctional layer 10 may include an n-type semiconductor layer.

The nitride semiconductor element 110 may include, for example, anitride semiconductor element of a gallium nitride (GaN) HEMT (HighElectron Mobility Transistor). In such a case, for example, thefunctional layer 10 has a stacked structure of an undopedAl_(z1)Ga_(1-z1)N (0≦z1≦1) layer not Including an impurity, and anundoped or n-type Al_(z2)Ga_(1-z2)N (0≦z2≦1 and z1<z2) layer.

A GaN layer 11 i (an undoped GaN layer) may be provided on the stackedbody 50 (e.g., between the stacked body 50 and the functional layer 10).Compressive strain (stress) is formed in the GaN layer 11 i and thecracks are suppressed further by providing the GaN layer 11 i (theundoped GaN layer). The GaN layer 11 i may be a doped semiconductorlayer such as an n-type semiconductor layer.

In FIG. 1B, FIG. 1C, and FIG. 1D, the vertical axis is the Z-axisdirection position.

The horizontal axis of FIG. 1B is the Al composition ratio C_(Al). Asillustrated in FIG. 1B, the Al composition ratio C_(Al) of the stackedbody 50 is substantially 0 in the GaN intermediate layer 51, issubstantially 1 in the high Al composition layer 52, and is higher than0 and lower than 1 in the low Al composition layer 53.

The horizontal axis of FIG. 1C is the growth temperature GT. AsIllustrated in FIG. 1C, for example, the growth temperature GT of theGaN Intermediate layer 51 is high. By the GaN intermediate layer 51being high, the lattice relaxation can be suppressed; and thecompressive stress formed in the GaN intermediate layer 51 can beIncreased. For example, it is favorable for the growth temperature GT tobe not less than 1000° C. and not more than 1200° C., e.g., about 1130°C.

The growth temperature GT of the high Al composition layer 52 is low.For example, it is favorable for the growth temperature GT of the highAl composition layer 52 to be not less than 500° C. and not more than1050° C., e.g., about 800° C. It is more favorable to be not less than600° C. and not more than 850° C. In the case where the growthtemperature GT of the high Al composition layer 52 is lower than 500°C., impurities are introduced easily. Also, cubic crystal AlGaN and thelike grow; and crystal dislocations undesirably occur excessively. Thecrystal quality of the high Al composition layer 52 undesirably degradesexcessively. In the case where the growth temperature GT of the high Alcomposition layer 52 is higher than 1050° C., the lattice relaxationdoes not occur easily. Therefore, the strain is not relaxed; and tensilestrain is easily introduced to the high Al composition layer 52.Further, the compressive stress is not applied appropriately whenforming the low Al composition layer 53 and/or the functional layer 10on the high Al composition layer 52; and cracks occur easily in thecooling process after the crystal growth. Conversely, the latticerelaxation of the high Al composition layer 52 occurs easily in the casewhere the growth temperature GT of the high Al composition layer 52 is,for example, 800° C. As a result, the spacing of the high Al compositionlayer 52 approaches the value of the lattice spacing of unstrainedstate. In other words, the tensile strain from the GaN intermediatelayer 51 and/or foundation layer 61 is not easily applied in the initialformation of the high Al composition layer 52. Thus, the high Alcomposition layer 52 having lattice relaxation is formed on the GaNintermediate layer 51.

The growth temperature GT of the low Al composition layer 53 is high.For example, it is favorable for the growth temperature GT of the low Alcomposition layer 53 to be not less than 800° C. and not more than 1200°C., e.g., 1130° C.

The horizontal axis of FIG. 1D is the lattice spacing Ld along thea-axis. FIG. 1D illustrates lattice spacing dg along the a-axis ofunstrained GaN (e.g., 0.3189 nm) and lattice spacing da along the a-axisof unstrained AlN (e.g., 0.3112 nm). The lattice spacing da along thea-axis (the first axis) of the Al_(x1)Ga_(1-x1)N (0<x1≦1) whenunstrained corresponds to the lattice constant along the a-axis (thefirst axis) of the Al_(x1)Ga_(1-x1)N (0<x1≦1). For example, in the casewhere the high Al composition layer 52 is AlN, the lattice spacing daalong the a-axis (the first axis) of the unstrained high Al compositionlayer 52 corresponds to the lattice constant along the a-axis (the firstaxis) of AlN. The lattice spacing dg of unstrained GaN is larger thanthe lattice spacing da of unstrained AlN.

As illustrated in FIG. 1D, the lattice spacing along the a-axis (thefirst axis) of the GaN intermediate layer 51 is large; and the latticespacing along the a-axis (the first axis) of the high Al compositionlayer 52 is small. For example, the actual lattice spacing along thea-axis (the first axis) of the GaN intermediate layer 51 is smaller thanthe lattice spacing dg along the a-axis (the first axis) of unstrainedGaN; and the actual lattice spacing along the a-axis (the first axis) ofthe high Al composition layer 52 is, for example, larger than thelattice spacing da along the a-axis (the first axis) of theAl_(x1)Ga_(1-x1)N (0<x1≦1) when unstrained. In the case where the highAl composition layer 52 is AlN, for example, the actual lattice spacingof the high Al composition layer 52 is larger than the lattice spacingda along the a-axis (the first axis) of unstrained AlN. In other words,in the stacked body 50, the lattice spacing along the a-axis (the firstaxis) is largest at the GaN intermediate layer 51 and decreases abruptlyat the high Al composition layer 52. The lattice spacing along thea-axis (the first axis) of the low Al composition layer 53 is the sameas or larger than the lattice spacing along the a-axis (the first axis)of the high Al composition layer 52.

When the actual lattice spacing along the a-axis (the first axis) of theGaN intermediate layer 51 is smaller than the lattice spacing dg alongthe a-axis (the first axis) of unstrained GaN, a compressive strain isapplied to the first intermediate GaN layer 51.

When the third lattice spacing of the first high Al composition layer 52is larger than the second lattice spacing (the Al_(x1)Ga_(1-x1)N(0<x1≦1) when unstrained), a tensile strain is applied to the first highAl composition layer 52.

It is favorable for the lattice spacing along the a-axis (the firstaxis) of the GaN intermediate layer 51 to be small because thecompressive stress that is applied to the GaN intermediate layer 51increases as the lattice spacing along the a-axis (the first axis) ofthe GaN intermediate layer 51 decreases. Additionally, the latticespacing of the high Al composition layer 52 that is formed on the GaNintermediate layer 51 decreases. The lattice spacing along the a-axis(the first axis) of the GaN intermediate layer 51 changes due to, forexample, the ammonia partial pressure when forming the GaN intermediatelayer 51. For example, the lattice spacing along the a-axis (the firstaxis) of the GaN Intermediate layer 51 decreases as the ammonia partialpressure increases. For example, it is favorable for the ammonia partialpressure to be not less than 0.2 and not more than 0.5. The latticespacing along the a-axis (the first axis) of the GaN intermediate layer51 changes due to, for example, the ratio (the V/III ratio) of thesource-material gas of the group V atoms and the source-material gas ofthe group III atoms when forming the GaN Intermediate layer 51. Forexample, the lattice spacing along the a-axis (the first axis) of theGaN intermediate layer 51 decreases as the V/III ratio increases. Forexample, it is favorable for the ammonia partial pressure to be not lessthan 2000 and not more than 8000.

Here, a relaxation rate α (a lattice relaxation rate) is introduced as aparameter corresponding to the level of the relaxation of the strain(the relaxation of the lattice) of the crystal.

The lattice relaxation of the high Al composition layer 52 occurs whenthe high Al composition layer 52 is grown on the GaN intermediate layer51. The relaxation rate α of the Al_(x1)Ga_(1-x1)N (0<x1≦1) which is thehigh Al composition layer 52 is the ratio of the absolute value of thedifference between the lattice spacing dg along the first axis (e.g.,the a-axis) of unstrained GaN and an actual lattice spacing Da along thefirst axis (e.g., the a-axis) of the high Al composition layer 52 (thefirst high Al composition layer 52 a) to the absolute value of thedifference between the lattice spacing dg along the first axis (e.g.,the a-axis) of unstrained GaN and the lattice spacing da along the firstaxis (e.g., the a-axis) of the Al_(x1)Ga_(1-x1)N (0<x1≦1) whenunstrained. In other words, the relaxation rate α of the high Alcomposition layer 52 is |dg−Da|/|dg−da|. The lattice spacing dg alongthe first axis (e.g., the a-axis) of unstrained GaN corresponds to thelattice constant of GaN. The lattice spacing da along the first axis(e.g., the a-axis) of the Al_(x1)Ga_(1-x1)N (0<x1≦1) when unstrainedcorresponds to the lattice constant along the first axis (e.g., thea-axis) of the Al_(x1)Ga_(1-x1)N (0<x1≦1).

For example, the lattice spacing (the lattice constant) along the firstaxis of Al_(x1)Ga_(1-x1)N when unstrained Is a value calculated usingVegard's law from the lattice spacing (the lattice constant) along thefirst axis of unstrained AlN and the lattice spacing (the latticeconstant) along the first axis of unstrained GaN.

The relaxation rate α of the high Al composition layer 52 changes due tothe growth temperature GT when the high Al composition layer 52 isgrown. Further, the relaxation rate α changes due to the growth rate,the ratio (the V/III ratio) of the source-material gas of the group Vatoms to the source-material gas of the group III atoms, the ammoniapartial pressure, etc. The V/III ratio is the ratio of the number of theatoms of the group V element supplied per unit time to the number of theatoms of the group III element supplied per unit time. The ammoniapartial pressure is the ratio of the pressure of the ammonia to thepressure of the entire gas used in the film formation.

FIG. 2A to FIG. 2D are graphs illustrating characteristics of thenitride semiconductor element.

These drawings Illustrate an example of the change of the relaxationrate α as the growth temperature GT, a growth rate GR, the V/III ratio(V/III), and an ammonia partial pressure Pp are changed in the formationwhen the AlN is grown in the case where the high Al composition layer 52is AlN.

FIG. 2A illustrates the change of the relaxation rate α as the growthtemperature GT is changed in the case where the V/III ratio is 11300,the growth rate GR is 3.9 nm/minute (min), and the ammonia partialpressure Pp is 0.06. As illustrated in FIG. 2A, for example, therelaxation rate α of the high Al composition layer 52 is 0.43 in thecase where the growth temperature GT is 1100° C. when the AlN which isthe high Al composition layer 52 is grown. In the case where the growthtemperature GT is 650° C., the relaxation rate α is 0.71. Thus, therelaxation rate α Increases when the growth temperature GT is low. Toincrease the relaxation rate α, it is favorable for the growthtemperature GT of the formation to be lower than the growth temperatureGT of the GaN intermediate layer 51.

FIG. 2B illustrates the change of the relaxation rate α as the growthrate GR Is changed in the case where the V/III ratio is 11300, thegrowth temperature GT is 800° C., and the ammonia partial pressure Pp is0.06. As illustrated In FIG. 2B, for example, the relaxation rate α is0.35 in the case where the growth rate GR of the AlN which is the highAl composition layer 52 is 8.82 nm/minute. The relaxation rate α is 0.57in the case where the growth rate GR is 3.9 nm/minute. Thus, therelaxation rate α increases as the growth rate GR decreases. To increasethe relaxation rate α, it is favorable for the growth rate of theformation to be less than the growth rate of the GaN intermediate layer51. For example, it is favorable to be not less than 2 nm/minute and notmore than 10 nm/minute. It is more favorable to be not less than 3nm/minute and not more than 8 nm/minute.

FIG. 2C illustrates the change of the relaxation rate α as the V/IIIratio Is changed in the case where the growth temperature GT is 800° C.,the growth rate GR Is 3.9 nm/minute, and the ammonia partial pressure Ppis 0.06. As Illustrated in FIG. 2C, for example, the relaxation rate αis 0.44 in the case where the V/III ratio is 1800 when the AlN which isthe high Al composition layer 52 is grown. In the case where the V/IIIratio is 22600, the relaxation rate α is 0.72. Thus, the relaxation rateα Increases when the V/III ratio is large. To Increase the relaxationrate α, for example, it is favorable for the V/III ratio to be not lessthan 1500 and not more than 100000. It is more favorable to be not lessthan 10000 and not more than 50000. It is difficult for the high Alcomposition layer 52 to be sufficiently relaxed when the V/III ratiobecomes smaller than 1500. When the V/III ratio becomes larger than100000, the vapor phase reaction between the ammonia which is thesource-material gas of the group V atoms and the aluminum which is thesource-material gas of the group III atoms becomes excessive; and thecrystal quality of the high Al composition layer 52 decreases.

FIG. 2D Illustrates the change of the relaxation rate α as the ammoniapartial pressure Pp is changed in the case where the growth temperatureGT is 800° C., the growth rate GR is 3.9 nm/minute, and the V/III ratiois 11300. As illustrated in FIG. 2D, for example, the relaxation rate αis 0.43 in the case where the ammonia partial pressure Pp is 0.009 whengrowing the AlN which is the high Al composition layer 52. In the casewhere the ammonia partial pressure Pp is 0.111, the relaxation rate α is0.72. Thus, the relaxation rate α increases when the ammonia partialpressure Pp is large. To increase the relaxation rate α, it is favorablefor the ammonia partial pressure Pp to be, for example, not less than0.01 and not more than 0.5. It is more favorable to be not less than0.04 and not more than 0.3. It is difficult for the high Al compositionlayer 52 to be sufficiently relaxed when the ammonia partial pressure Ppbecomes smaller than 0.01. The vapor phase reaction between the ammoniawhich is the source-material gas of the group V atoms and the aluminumwhich is the source-material gas of the group III atoms becomesexcessive when the ammonia partial pressure Pp becomes larger than 0.5;and the crystal quality of the high Al composition layer 52 decreases.

An increase of the relaxation rate α corresponds to a decrease of theactual lattice spacing Da along the a-axis (the first axis) of the highAl composition layer 52.

In the case where the lattice relaxation occurs completely in the highAl composition layer 52 and the actual lattice spacing Da along thea-axis (the first axis) of the high Al composition layer 52 is equal tothe lattice spacing da along the a-axis (the first axis) of theAl_(x1)Ga_(1-x1)N (0<x1≦1) when unstrained, the crystal information ofthe GaN intermediate layer 51 cannot be acquired; fluctuation of thecrystal axis occurs; and the crystal quality drastically degrades. Also,misfit dislocations increase due to the lattice relaxation; and thecrystal quality degrades. Accordingly, it is favorable for the actuallattice spacing Da along the a-axis (the first axis) of the high Alcomposition layer 52 to be larger than the lattice spacing da along thea-axis (the first axis) of the Al_(x1)Ga_(1-x1)N (0<x1≦1) whenunstrained. That is, it is preferable that the high Al composition layer52 has a tensile stress. In other words, it is preferable that a tensilestress is applied to the high Al composition layer 52.

Continuing, the low Al composition layer 53 is formed on the high Alcomposition layer 52.

In the embodiment, the Al composition ratio of the low Al compositionlayer 53 is not more than the relaxation rate α of the high Alcomposition layer 52. In such a case, the lattice constant along thefirst axis (e.g., the a-axis) of the low Al composition layer 53 islarger than the actual lattice spacing of the high Al composition layer52. The low Al composition layer 53 grows while being subjected to thecompressive strain to have lattice matching with the lattice of the highAl composition layer 52. Therefore, an actual lattice spacing Dag alongthe first axis (e.g., the a-axis) of the low Al composition layer 53 isequal to or larger than the actual lattice spacing Da along the firstaxis (e.g., the a-axis) of the high Al composition layer 52.

Conversely, in the case where the Al composition ratio of the low Alcomposition layer 53 is larger than the relaxation rate α of the high Alcomposition layer 52, the lattice constant along the first axis (e.g.,the a-axis) of the low Al composition layer 53 is smaller than thelattice spacing of the high Al composition layer 52. Therefore, the lowAl composition layer 53 grows while being subjected to the tensilestrain; and the lattice spacing along the first axis (e.g., the a-axis)of the low Al composition layer 53 is smaller than the actual latticespacing Da along the first axis (e.g., the a-axis) of the high Alcomposition layer 52. Therefore, the tensile strain occurs; and thecracks occur easily.

In other words, in the case where the low Al composition layer 53 havingan inappropriate Al composition ratio is formed on the high Alcomposition layer 52, the compressive strain is not formed; and thecracks cannot be suppressed. By forming the low Al composition layer 53having an appropriate Al composition ratio that reflects the relaxationrate α of the high Al composition layer 52, the appropriate compressivestrain is formed; and the cracks can be suppressed. In other words, goodcharacteristics are obtained by forming the low Al composition layer 53with an Al composition ratio that is not more than the relaxation rate αof the high Al composition layer 52.

For example, it is favorable for the thickness of the low Al compositionlayer 53 to be not less than 5 nm and not more than 100 nm. In the casewhere the thickness of the low Al composition layer 53 is thinner than 5nm, it is difficult to obtain the suppression effect of the cracks andthe reduction effect of the dislocations. In the case where thethickness of the low Al composition layer 53 is thicker than 100 nm, notonly does the effect of reducing the dislocations saturate, but also thecracks occur more easily. It is more favorable for the thickness of thelow Al composition layer 53 to be less than 50 nm. By the thickness ofthe low Al composition layer 53 being less than 50 nm, the cracks andthe dislocation density can be reduced effectively.

For example, in the case where the high Al composition layer 52 is AlNand the low Al composition layer 53 is Al_(x1)Ga_(1-x1)N, theAl_(x1)Ga_(1-x1)N layer is formed to have lattice matching with theactual lattice spacing of the AlN layer and grows while being subjectedto the strain in the state (i.e., the initial growth) in which theAl_(x1)Ga_(1-x1)N layer is thin when growing the Al_(x1)Ga_(1-x1)N layeron the AlN layer. Then, as the growth of the Al_(x1)Ga_(1-x1)Nprogresses, the strain is gradually relaxed; and the lattice spacing ofthe Al_(x1)Ga_(1-x1)N that is grown approaches the lattice spacing ofthe Al_(x1)Ga_(1-x1)N in the state in which the strain is not applied.The Al_(x1)Ga_(1-x1)N grows while being subjected to compressive strainand the compressive strain is stored in the substrate 40 surface byforming the Al_(x1)Ga_(1-x1)N layer on an AlN layer such that theAl_(x1)Ga_(1-x1)N layer has lattice spacing that is smaller than thelattice constant of the Al_(x1)Ga_(1-x1)N, where the lattice constant ofthe Al_(x1)Ga_(1-x1)N is larger than the actual lattice spacing of theAlN layer. As a result, warp having an upward-protruding configuration(a convex configuration) occurs in the substrate 40. By pre-storing thecompressive strain during the crystal growth, the occurrence of thecracks that occur due to the coefficient of thermal expansion differencewhen cooling after the crystal growth can be suppressed. In addition tosuppressing the cracks, the dislocations can be reduced by setting theAl composition ratio of the low Al composition layer 53 to a value thatreflects the relaxation rate α of the high Al composition layer 52 andby setting the film thickness to an appropriate value (the valuesrecited above).

The low Al composition layer 53 may include multiple layers. In such acase as well, the cracks can be suppressed by the Al composition ratioof the low Al composition layer 53 being not more than the relaxationrate α of the high Al composition layer 52. For example, the Alcomposition ratio of the low Al composition layer 53 may change In astep configuration that decreases in stages along the growth directionfrom the substrate 40 side or a gradually decreasing configuration alongthe growth direction. By such a configuration, the lattice relaxation ofthe low Al composition layer 53 can be suppressed; and the compressivestress formed in the low Al composition layer 53 can be increased. Also,the dislocations that reach the functional layer 10 can be reducedbecause bending occurs for the dislocations that occur at the substrateinterface between the low Al composition layer 53 and the layer formedon the low Al composition layer 53.

As illustrated in FIG. 1C, the growth temperature GT of the low Alcomposition layer 53 is, for example, about 1130° C. In the case wherethe growth temperature GT of the low Al composition layer 53 is higherthan the growth temperature GT of the high Al composition layer 52 bynot less than 80° C., the effect of growing the low Al composition layer53 to have lattice matching with the actual lattice spacing of the highAl composition layer 52 can be greater. For example, when the growthtemperature GT of the low Al composition layer 53 is not less than 880°C., the thickness at which the growth has lattice matching increases. Asa result, the compressive strain occurs easily; and the cracks aresuppressed easily. Also, the reduction effect of the dislocations isgreater. It is favorable for the growth temperature GT of the low Alcomposition layer 53 to be not less than the growth temperature GT ofthe GaN Intermediate layer 51. Thereby, the flatness of the low Alcomposition layer 53 improves; and the crystallinity of the nitridesemiconductor layer (e.g., the functional layer 10) that is formed onthe low Al composition layer 53 improves.

For example, it is favorable for the total thickness of the high Alcomposition layer 52, the low Al composition layer 53, and the GaNintermediate layer 51 to be not less than 50 nm and not more than 2000nm. In the case where the total thickness is less than 50 nm, thecompressive stress is hard to be generated and the number of thestacking to suppress the crack increases. In the case where the numberof stacks of these layers Is large, the number of the heating processesand cooling processes for the growth temperature GT to obtain thedesired thickness of the stacked body 50 increases excessively. Thereby,the crystal quality decreases by the excessive change of thetemperature. Further, the productivity decreases. On the other hand, inthe case where the total thickness is thicker than 2000 nm, the latticerelaxation easily occurs. As a result, the storage of the compressivestrain is insufficient; and the cracks occur easily. It is morefavorable for the total thickness to be not less than 300 nm and lessthan 1000 nm. By the total thickness being not less than 300 nm and lessthan 1000 nm, a flat surface is obtained easily; and it is easy torealize the effect of reducing the cracks and the dislocations.

As shown in FIGS. 11A-11C, a δ-doped layer 50 dd of SI may be providedat a portion of the GaN intermediate layer 51. In an example shown inFIG. 11A, the δ-doped layer 50 dd is provided on the surface of the GaNIntermediate layer 51 on the high Al composition layer 52 side. In anexample shown In FIG. 11B, the δ-doped layer 50 dd is provided in theinterior of the GaN intermediate layer 51. In an example shown in FIG.11C, the δ-doped layer 50 dd is provided on the surface of the GaNintermediate layer 51 on the foundation layer 60 side.

As shown in FIG. 11D, the δ-doped layer 50 dd of Si may be provided at aportion of the high Al composition layer 52. In this case, the δ-dopedlayer 50 dd may be provided on the surface of the high Al compositionlayer 52 on the low Al composition layer 53 side. The δ-doped layer 50dd may be provided in the interior of the high Al composition layer 52.The δ-doped layer 50 dd may be provided on the surface of the high Alcomposition layer 52 on the GaN intermediate layer 51 side.

As shown in FIG. 11E, the δ-doped layer 50 dd of Si may be provided at aportion of the low Al composition layer 53. In this case, the δ-dopedlayer 50 dd may be provided on the surface of the low Al compositionlayer 53 on the GaN layer 11 i side. The δ-doped layer 50 dd may beprovided in the interior of the low Al composition layer 53. The δ-dopedlayer 50 dd may be provided on the surface of the low Al compositionlayer 53 on the high Al composition layer 52 side.

The δ-doped layer 50 dd may Include, for example, a layer that containsSi with a concentration of not less than 5×10¹⁷ cm⁻³ and not more than2×10¹⁹ cm⁻³. By providing the δ-doped layer 50 dd with such Siconcentration, the compressive stress of the GaN layer (for example, theGaN layer 11 i) formed on the δ-doped layer 50 dd is increased and thecrack can be suppressed more effectively.

Alternately, the δ-doped layer 50 dd may include, for example, a layerthat contains Si with a concentration of not less than 7×10¹⁹ cm⁻³ andnot more than 5×10²⁰ cm⁻³. By providing these δ-doped layers 50 dd,shielding of the dislocations or bending of the dislocations occur atthe δ-doped layer 50 dd; and the dislocations that would reach thesemiconductor layer (e.g., the functional layer 10) that is formed onthe δ-doped layer 50 dd can be reduced more effectively.

The concentration of Si in the δ-doped layer 50 dd can be measured by asecondary ion secondary mass spectrometry (SIMS). In the SIMSmeasurement of the Si concentration of the δ-doped layer 50 dd with thinthickness, there is case in which the measured result of the Siconcentration shows a spreading profile in the thickness direction. Insuch a case, the Si concentration can be obtained from a value of Sisheet density. The Si sheet density is a value obtained by anintegration of Si concentration in the depth direction (in the thicknessdirection). For example, the sheet density can be calculated to be avalue of a total sum of Si atoms obtained by the integration in the areaof the thickness of 200 nm in the thickness direction with a center forthe Si concentration peak. For example, the Si concentration measured bySIMS of about 2×10²⁰ cm⁻³ corresponds to the sheet density of about1×10¹⁵ cm⁻². Therefore, the Si concentration of the δ-doped layer 50 ddnot less than 5×10¹⁷ cm⁻³ and not more than 2×10¹⁹ cm⁻³ corresponds thesheet density not less than 2.5×10¹² cm⁻² and not more than 1×10¹⁴ cm⁻².The Si concentration of the δ-doped layer 50 dd not less than 7×10¹⁹cm⁻³ and not more than 5×10²⁰ cm⁻³ corresponds the sheet density notless than 3.5×10¹⁴ cm⁻² and not more than 2.5×10¹⁵ cm⁻².

For example, the thickness of the δ-doped layer 50 dd is not less than0.3 nm and not more than 200 nm. However, the concentration and thethickness are not limited thereto. The δ-doped layer 50 dd may include aSiN layer in which a part of Si is bonded to nitrogen. The δ-doped layer50 dd may be formed in non-continuous configuration as well as incontinuous configuration.

In the nitride semiconductor element 110 according to the embodiment,the stacked body 50 is provided between the functional layer 10 and thefoundation layer 60. The stacked body 50 has a structure in which theGaN intermediate layer 51, the high Al composition layer 52, and the lowAl composition layer 53 are stacked in this order. The composition ratioof the Al of the low Al composition layer 53 is not more than therelaxation rate α of the high Al composition layer 52, where therelaxation rate α of the high Al composition layer is the ratio of thedifference between the lattice spacing dg along the first axis (e.g.,the a-axis) of unstrained GaN and the actual lattice spacing Da alongthe first axis (e.g., the a-axis) of the high Al composition layer 52 tothe absolute value of the difference between the lattice spacing dgalong the first axis (e.g., the a-axis) of unstrained GaN and thelattice spacing da along the first axis (e.g., the a-axis) of theAl_(x1)Ga_(1-x1)N (0<x1≦1) when unstrained. Thereby, the compressivestress is applied in the crystal growth; and the effect of suppressingthe occurrence of the cracks is obtained. Also, the effect of reducingthe dislocations is obtained. Therefore, the cracks, the dislocations,etc., of the functional layer 10 are reduced. According to the nitridesemiconductor element 110, a high-quality nitride semiconductor elementthat is formed on the substrate 40 (e.g., the silicon substrate) and hasfew cracks is obtained.

FIG. 3A to FIG. 3D are schematic views illustrating the configuration ofanother nitride semiconductor element according to the first embodiment.

FIG. 3A is a schematic cross-sectional view illustrating theconfiguration of the nitride semiconductor element 120 according to theembodiment. FIG. 3B to FIG. 3D are graphs illustrating the Alcomposition ratio (C_(Al)), the growth temperature GT, and the latticespacing Ld along the a-axis, respectively, of the stacked intermediatelayer.

As illustrated in FIG. 3A, the nitride semiconductor element 120includes the foundation layer 60, the stacked body 50, and thefunctional layer 10. The configurations of the foundation layer 60 andthe functional layer 10 are similar to those described in regard to thenitride semiconductor element 110, and a description is thereforeomitted. In such a case as well, the GaN layer 11 i (the undoped GaNlayer) may be provided between the stacked body 50 and the functionallayer 10.

The configuration of the stacked body 50 of the nitride semiconductorelement 120 is different from that of the nitride semiconductor element110. The stacked body 50 will now be described.

In the nitride semiconductor element 120, the stacked body 50 includesthe first stacked intermediate layer 50 a and a second stackedintermediate layer 50 b. The first stacked Intermediate layer 50 a isprovided between the foundation layer 60 and the functional layer 10.The second stacked intermediate layer 50 b is provided between the firststacked Intermediate layer 50 a and the functional layer 10.

The first stacked intermediate layer 50 a includes the first GaNintermediate layer 51 a provided on the foundation layer 60, the firsthigh Al composition layer 52 a provided on the first GaN intermediatelayer 51 a, and the first low Al composition layer 53 a provided on thefirst high Al composition layer 52 a.

The second stacked intermediate layer 50 b includes a second GaNIntermediate layer 51 b provided on the first stacked intermediate layer50 a, a second high Al composition layer 52 b provided on the second GaNintermediate layer 51 b, and a second low Al composition layer 53 bprovided on the second high Al composition layer 52 b.

The configurations of the first and second GaN Intermediate layers 51 aand 51 b are similar to the configuration of the GaN intermediate layer51 described in regard to the nitride semiconductor element 110. Theconfigurations of the first and second high Al composition layers 52 aand 52 b are similar to the configuration of the high Al compositionlayer 52 described in regard to the nitride semiconductor element 110.The configurations of the first and second low Al composition layers 53a and 53 b are similar to the configuration of the low Al compositionlayer 53 described in regard to the nitride semiconductor element 110.

The first high Al composition layer 52 a may include Al_(x1)Ga_(1-x1)N(0<x1≦1).

The first low Al composition layer 53 a may include Al_(y1)Ga_(1-y1)N(0<y1<1 and y1<x1).

The second high Al composition layer 52 b may include Al_(x2)Ga_(1-x2)N(0<x2≦1).

The second low Al composition layer 53 b may include Al_(y2)Ga_(1-y2)N(0<y2<1 and y2<x2).

The Al composition ratio of the first low Al composition layer 53 a isnot more than the relaxation rate of the first high Al composition layer52 a (a first relaxation rate αa, i.e., the ratio of the absolute valueof the difference between the lattice spacing dg along the first axis(e.g., the a-axis) of unstrained GaN and an actual lattice spacing Da1along the first axis (e.g., the a-axis) of the first high Al compositionlayer 52 a to the absolute value of the difference between the latticespacing dg along the first axis (e.g., the a-axis) of unstrained GaN andthe lattice spacing da along the first axis (e.g., the a-axis) of theAl_(x1)Ga_(1-x1)N (0<x1≦1) when unstrained).

The Al composition ratio of the second low Al composition layer 53 b isnot more than the relaxation rate of the second high Al compositionlayer 52 b (a second relaxation rate αb, i.e., the ratio of the absolutevalue of the difference between the lattice spacing dg along the firstaxis (e.g., the a-axis) of unstrained GaN and an actual lattice spacingDa2 along the first axis (e.g., the a-axis) of the second high Alcomposition layer 52 b to the absolute value of the difference betweenthe lattice spacing dg along the first axis (e.g., the a-axis) ofunstrained GaN and lattice spacing da2 along the first axis (e.g., thea-axis) of the Al_(x2)Ga_(1-x2)N (0<x2≦1) when unstrained.

Two sets (periods) of the set that includes the GaN intermediate layer,the high Al composition layer, and the low Al composition layer areprovided In the nitride semiconductor element 120. The embodiment is notlimited thereto; and the number of the sets (the periods) may be threeor more.

In the nitride semiconductor element 120 as well, a nitridesemiconductor element that is formed on the substrate (e.g., the siliconsubstrate) with few cracks and dislocations is obtained.

In the nitride semiconductor element 120, the configuration of thesecond stacked Intermediate layer 50 b may be different from that of thefirst stacked Intermediate layer 50 a. By repeating the forming thestacked intermediate layer 50 multiply, the relaxation of the strainbecomes easily suppressed and the compressive strain Is increased. Onthe functional layer 10 s side of the stacked body, the thickness inwhich the layer can be formed with applying the compressive stress isincreased. Therefore, for example, the thickness of the second stackedintermediate layer 50 b may be thicker than the thickness of the firststacked Intermediate layer 50 a. For example, the thickness of thesecond GaN Intermediate layer 51 b may be thicker than the thickness ofthe first GaN Intermediate layer 51 a. The effect of further reducingthe cracks and/or dislocations is obtained by changing the structure tocorrespond to the change of the amount of the strain that is stored inthe stacking.

As Illustrated in FIG. 3D, the lattice spacing along the a-axis of thefirst and second GaN intermediate layers 51 a and 51 b is large; and thelattice spacing along the a-axis of the first and second high Alcomposition layers 52 a and 52 b is small. For example, the actuallattice spacing along the a-axis of the first and second GaNintermediate layers 51 a and 51 b is smaller than the lattice spacing dgalong the a-axis of unstrained GaN. For example, the actual latticespacing along the a-axis of the first high Al composition layer 52 a islarger than the lattice spacing da1 along the a-axis of theAl_(x1)Ga_(1-x1)N (0<x1≦1) when unstrained. For example, the actuallattice spacing along the a-axis of the second high Al composition layer52 b is larger than the lattice spacing da2 along the a-axis of theAl_(x2)Ga_(1-x2)N (0<x2≦1) when unstrained. In other words, the latticespacing along the a-axis of the first and second stacked intermediatelayers 50 a and 50 b is greatest at the first and second GaNintermediate layers 51 a and 51 b and decreases abruptly at the firstand second high Al composition layers 52 a and 52 b. The lattice spacingalong the a-axis of the first and second low Al composition layers 53 aand 53 b is the same as or larger than the lattice spacing along thea-axis of the first and second high Al composition layers 52 a and 52 b.

The formation conditions, the characteristics, etc., of the first GaNintermediate layer 51 a, the first high Al composition layer 52 a, andthe first low Al composition layer 53 a of the first stackedIntermediate layer 50 a are similar to the formation conditions and thecharacteristics of the high Al composition layer 52, the low Alcomposition layer 53, and the GaN intermediate layer 51 of the stackedbody 50 described in regard to the nitride semiconductor element 110.

The second GaN intermediate layer 51 b of the second stackedintermediate layer 50 b is formed on the first stacked Intermediatelayer 50 a.

As illustrated In FIG. 3C, the growth temperature GT of the second GaNintermediate layer 51 b is, for example, about 1130° C. For example, itis favorable for the thickness of the second GaN intermediate layer 51 bto be not less than 100 nm and not more than 1000 nm, e.g., about 300nm.

The second high Al composition layer 52 b is formed on the second GaNintermediate layer 51 b. For example, it is favorable for the thicknessof the second high Al composition layer 52 b to be not less than 5 nmand not more than 100 nm, e.g., about 12 nm. It is more favorable forthe thickness of the second high Al composition layer 52 b to be notless than 10 nm and not more than 30 nm. The degradation of the crystalquality is suppressed further in the case where the thickness of thesecond high Al composition layer 52 b is not more than 30 nm. Forexample, it is favorable for the growth temperature GT of the secondhigh Al composition layer 52 b to be not less than 500° C. and not morethan 1050° C., e.g., about 800° C. The lattice relaxation of the secondhigh Al composition layer 52 b occurs easily by forming at such atemperature.

Thereby, as illustrated in FIG. 3D, compared to the lattice spacingalong the a-axis of the first GaN Intermediate layer 51 a and thelattice spacing along the a-axis of the second GaN Intermediate layer 51b, the actual lattice spacing Ld along the a-axis of the second high Alcomposition layer 52 b abruptly approaches the lattice spacing (thelattice constant) of the Al_(x2)Ga_(1-x2)N (0<x2≦1) in the unstrainedstate. Therefore, from the initial formation stage of the second high Alcomposition layer 52 b, the tensile strain from the first GaNIntermediate layer 51 a which is used to form the foundation is noteasily applied. As a result, the second high Al composition layer 52 bcan be formed in the state of not being easily affected by the strainfrom the first GaN intermediate layer 51 a used to form the foundation.Thus, the second high Al composition layer 52 b that has abrupt latticerelaxation is formed on the first GaN intermediate layer 51 a.

Continuing, the second low Al composition layer 53 b that has acomposition ratio of Al that is not more than the second relaxation rateαb of the second high Al composition layer 52 b is formed on the secondhigh Al composition layer 52 b. For example, it is favorable for thethickness of the second low Al composition layer 53 b to be not lessthan 5 nm and not more than 100 nm. It Is more favorable for thethickness of the second low Al composition layer 53 b to be less than 50nm.

The cracks and the dislocation density can be reduced effectively by thethickness of the second low Al composition layer 53 b being less than 50nm. The thickness of the second low Al composition layer 53 b is, forexample, about 25 nm.

For example, in the case where the second high Al composition layer 52 bis AlN and the second low Al composition layer 53 b isAl_(x1)Ga_(1-x1)N, the Al_(x1)Ga_(1-x1)N is formed to have latticematching with the actual lattice spacing of the AlN layer in the statein which the thickness Is thin, that is, in the initial growth, andgrows while being subjected to the strain. Then, the strain is graduallyrelaxed as the growth of the Al_(x1)Ga_(1-x1)N progresses; and theAl_(x1)Ga_(1-x1)N approaches the lattice spacing of theAl_(x1)Ga_(1-x1)N In the unstrained state.

As illustrated in FIG. 3B, for example, it is favorable for the Alcomposition ratio C_(Al) of the second low Al composition layer 53 b tobe not less than 0.1 and not more than 0.9, e.g., about 0.5. Forexample, an Al_(0.5)Ga_(0.5)N layer is used as the second low Alcomposition layer 53 b.

As illustrated in FIG. 3C, for example, it is favorable for the growthtemperature GT of the second low Al composition layer 53 b to be notless than 800° C. and not more than 1200° C., e.g., about 1130° C. Inthe case where the growth temperature GT of the second low Alcomposition layer 53 b is higher than the growth temperature GT of thesecond high Al composition layer 52 b by not less than 80° C., theeffect of growing the second high Al composition layer 52 b to havelattice matching with the actual lattice spacing is greater. Also, theeffect of reducing the dislocations is greater. For example, when thegrowth temperature GT of the second low Al composition layer 53 b is notless than 880° C., the thickness at which the growth has latticematching increases.

Thus, the stacked body 50 may have a structure in which the high Alcomposition layer 52, the low Al composition layer 53, and the GaNintermediate layer 51 are multiply stacked periodically in this order.Thereby, the compressive stress is applied in the crystal growth; andthe effect of suppressing the occurrence of the cracks is greater. Also,the effect of reducing the dislocations is greater. Therefore, thecracks, the dislocations, etc., of the functional layer 10 are reducedfurther.

An example of the characteristics of the nitride semiconductor elementof the embodiment will now be described.

The inventor of the application constructed the following samples.

The foundation layer 60 was formed on the substrate 40 (a siliconsubstrate). After forming the GaN intermediate layer 51 on the GaNfoundation layer 61 (an undoped GaN layer) of the foundation layer 60with a thickness of 300 nm at 1090° C., an AlN layer was formed with athickness of 12 nm at 800° C. The AlN layer corresponds to the high Alcomposition layer 52.

Continuing, an Al_(z)Ga_(1-z)N layer was formed on the AlN layer with athickness of 25 nm at 1130° C. This layer corresponds to the low Alcomposition layer 53. Here, the four types of the Al composition ratiosz of the Al_(z)Ga_(1-z)N layers of 0.2, 0.35, 0.5, and 0.7 were used.

Then, three more periods of the stacked body recited above were formed,where one period is the stacked body of the GaN layer, the AlN layer,and the Al_(z)Ga_(1-z)N layer. That is, the number of periods of thestacked bodies of the intermediate layers was four for the four types ofsamples constructed by the inventor.

Continuing, the undoped GaN layer 11 i was formed on the fourthAl_(z)Ga_(1-z)N layer with a thickness of 1 μm at 1090° C. Subsequently,an n-type GaN layer was formed with a thickness of 1 μm. Silicon wasused as the n-type impurity; and the impurity concentration was 5×10¹⁸cm⁻³. The n-type GaN layer was at least a portion of the functionallayer 10.

FIG. 4 is a graph illustrating the characteristics of the nitridesemiconductor elements.

FIG. 4 illustrates the characteristics relating to the curvature of thesubstrate when growing the samples that were constructed.

The horizontal axis of FIG. 4 is a thickness t_(AlGaN) (nm) when growingthe Al_(z)Ga_(1-z)N layer. The vertical axis illustrates a curvature Cv(km⁻¹=1000 m⁻¹) of the substrate 40 in the crystal growth. The curvatureCv is a value that substantially corresponds to the warp of thesubstrate 40. The curvature Cv of the substrate 40 is a value measuredby optical monitoring system in the crystal growth. The curvature Cvindicates the transition of the warp of the substrate 40 during thecrystal growth of the Al_(z)Ga_(1-z)N layer. For the curvature Cv inthis drawing, the warp of the substrate 40 is 0 when starting thecrystal growth. The curvature Cv having a positive value corresponds tothe state of protruding upward (warp having a recessed configuration,i.e. concave configuration). A negative value corresponds to the stateof protruding downward (warp having a protruding configuration, i.e.convex configuration). A positive curvature Cv corresponds to the warpof the substrate due to the tensile stress applied to the nitridesemiconductor crystal. A negative curvature Cv corresponds to the warpof the substrate 40 due to the compressive stress applied to the nitridesemiconductor crystal. FIG. 4 illustrates the characteristics of thefour types of samples having different Al composition ratios of theAl_(z)Ga_(1-z)N layer.

As illustrated in FIG. 4, the curvature Cv is positive in the case wherethe Al composition ratio z is 0.7. In other words, warp having arecessed configuration (convex configuration) occurs; and tensile stressis applied to the Al_(z)Ga_(1-z)N layer. The absolute value of thecurvature Cv Increases and the tensile stress increases as the thicknesst_(AlGaN) of the Al_(z)Ga_(1-z)N layer Increases.

The curvature Cv is negative when the Al composition ratio z is 0.2,0.35, or 0.5. In other words, warp having a protruding configurationoccurs; and compressive stress is applied to the Al_(z)Ga_(1-z)N layer.The absolute value of the curvature Cv increases as the thicknesst_(AlGaN) of the Al_(z)Ga_(1-z)N layer increases.

Although a slight compressive stress is applied in the initial growthwhen the Al composition ratio z is 0.5, saturation eventually occurs;and the curvature Cv of the substrate 40 does not change and is constanteven in the case where the thickness t_(AlGaN) is increased. This meansthat the stress substantially is not formed in the Al_(z)Ga_(1-z)Nlayer. In the case where the Al composition ratio z is small, i.e.,0.35, the compressive stress is higher and the warp having theprotruding configuration is greater than when the Al composition ratio zis 0.5. When the Al composition ratio z is 0.2, the compressive stressincreases further and the warp having the protruding configurationincreases further.

FIG. 5A to FIG. 5D are micrographs illustrating characteristics of thenitride semiconductor elements.

FIG. 5A to FIG. 5D are micrographs of the surface of the n-type GaNlayer (the layer used to form at least a portion of the functional layer10). FIG. 5A to FIG. 5D correspond to the four types of samples in whichthe Al composition ratios z of the Al_(z)Ga_(1-z)N layers are 0.2, 0.35,0.5, and 0.7, respectively.

As illustrated in FIG. 5D, many cracks occurred in the n-type GaN layerfor the sample in which the Al composition ratio z of theAl_(z)Ga_(1-z)N layer was 0.7 (the sample in which the tensile stresswas formed).

As illustrated in FIG. 5A to FIG. 5C, cracks were not observed in thecase where the Al composition ratio z was 0.2, 0.35, or 0.5. The cracksthat occur in the n-type GaN layer were reduced by the compressivestress being formed. In other words, it was found that the occurrence ofthe cracks can be greatly suppressed by suppressing the tensile strainapplied to the low Al composition layer 53 (the Al_(z)Ga_(1-z)N layer).

Thus, it was found that the stress applied to the low Al compositionlayer 53 has a relationship with the Al composition ratio of the low Alcomposition layer 53.

The actual lattice spacing along the a-axis at the formation temperatureof the high Al composition layer 52 can be calculated from the resultsof the change of the curvature of the substrate 40 in the growth of thehigh Al composition layer 52 measured by optical monitoring. In thisexample, the high Al composition layer 52 was AlN; and the latticespacing of the AlN was 0.3145 nm when converted to the actual latticespacing at room temperature. On the other hand, the lattice spacing dgalong the a-axis of unstrained GaN is, for example, 0.3189 nm; and thelattice spacing da along the a-axis of unstrained AlN is, for example,0.3112 nm. Accordingly, in this case, the relaxation rate α of the AlNwhich is the high Al composition layer 52 corresponds to 0.57.

FIG. 6A to FIG. 6D are schematic views illustrating the results of X-raydiffraction measurements of the nitride semiconductor elements.

These drawings are examples of reciprocal lattice mapping images of the(11-24) plane measured by X-ray diffraction measurements. The horizontalaxis is a reciprocal Qx of the lattice plane spacing of the (11-20)plane of a direction perpendicular to the growth direction. Qx is avalue that is proportional to the reciprocal of the lattice spacingalong the a-axis. The vertical axis is a reciprocal Qz of the latticeplane spacing of the (0004) plane of a direction parallel to the growthdirection. Qz is a value that is proportional to the reciprocal of thelattice spacing along the c-axis. These drawings illustrate the point ofa diffraction peak Pg of the (11-24) plane of the GaN (corresponding tothe reciprocal of the lattice spacing of the GaN) and the point of adiffraction peak Pa of the (11-24) plane of the AlN (corresponding tothe reciprocal of the lattice spacing of the AlN buffer layer 62). Thedotted line connecting these points illustrates the characteristic ofthe reciprocal of the lattice spacing corresponding to the Alcomposition ratio of the AlGaN layer according to Vegard's law.

These drawings illustrate a point P63 of the diffraction peak of the(11-24) plane due to the AlGaN foundation layer 63 and a point P53 ofthe diffraction peak of the (11-24) plane due to the low Al compositionlayer 53.

In these drawings, the case where the peaks (the point P63 and the pointP53) of the measurement results of the lattice spacing of the AlGaNlayer appear on the lower side of the dotted line corresponds to thecrystal having compressive strain (being subjected to compressivestress). The case where the peaks (the point P63 and the point P53) ofthe measurement results appear on the upper side of the dotted linecorresponds to the crystal having tensile strain (being subjected totensile stress).

FIG. 6A to FIG. 6D correspond to the four types of samples in which theAl composition ratios z of the Al_(z)Ga_(1-z)N layers are 0.2, 0.35,0.5, and 0.7, respectively.

FIG. 6A to FIG. 6D show that the point P63 of the peak due to the AlGaNfoundation layer 63 appears on the lower side of the dotted line for allof the samples. This shows that the AlGaN foundation layer 63 hascompressive strain (is subjected to compressive stress).

The stacked body 50 is formed on such an AlGaN foundation layer 63. Byforming the stacked body 50 on the AlGaN foundation layer 63, thecompressive stress formed in the stacked body 50 increases; the tensilestrain occurring in the cooling process after the crystal growthdecreases; and the effect of suppressing the cracks is greater.

In the case where the Al composition ratio z of the low Al compositionlayer 53 is 0.7 as illustrated in FIG. 6D, it can be seen that the peakappears on the upper side of the dotted line; and the low Al compositionlayer 53 has tensile strain (is subjected to tensile stress).

On the other hand, in the case where the composition ratio z of the Alof the low Al composition layer 53 is 0.5 as Illustrated in FIG. 6C, itcan be seen that the peak appears slightly on the lower side of thedotted line; and the low Al composition layer 53 has compressive strain(is subjected to compressive stress).

In the case where the Al composition ratio z of the low Al compositionlayer 53 is 0.35 as illustrated in FIG. 6B, the peak of the low Alcomposition layer 53 overlays the peak of the AlGaN foundation layer 63;and a distinct peak is not observed for the low Al composition layer 53.In other words, the peak of the low Al composition layer 53 has moveddownward from the state illustrated in FIG. 6C.

In the case where the Al composition ratio z of the low Al compositionlayer 53 is 0.2 as illustrated in FIG. 6A, the peak of the low Alcomposition layer 53 has moved downward from the state illustrated inFIG. 6B.

Thus, as the Al composition ratio z of the low Al composition layer 53decreases, the peak position appears more on the side of being subjectedto the compressive strain. These results are well-matched to the resultsof the stress change due to the curvature Cv in the growth illustratedin FIG. 4. Also, this is well-matched to the results of the crackdensity change illustrated in FIG. 5A to FIG. 5D.

In the case where the Al composition ratio z of the low Al compositionlayer 53 formed on the high Al composition layer 52 was 0.7, tensilestrain was formed in the low Al composition layer 53. In the case wherethe Al composition ratio z of the low Al composition layer 53 formed onthe high Al composition layer 52 was 0.5, compressive strain was formedin the low Al composition layer 53. From these results, it is consideredthat the lattice spacing along the a-axis of the high Al compositionlayer 52 is the lattice spacing along the a-axis of the AlGaN layer ofthe Al composition ratio in the range of not less than 0.5 and not morethan 0.7. This value substantially matches the value (0.57) of therelaxation rate α of the high Al composition layer 52 that wascalculated from the curvature change of the substrate 40 when growingthe high Al composition layer 52.

In X-ray diffraction measurements of a nitride semiconductor elementthat was constructed without providing the low Al composition layer 53,the Qx value of the diffraction peak of the (11-24) plane of the high Alcomposition layer 52 was confirmed at a position corresponding to theAlGaN layer in which the Al composition ratio is 0.54. In other words,it was found that the relaxation rate α of the high Al composition layer52 was about 0.54; and this substantially matches the results recitedabove.

In other words, it was found that the tensile stress occurs in the lowAl composition layer 53 (the low Al composition layer 53 has tensilestrain) in the case where the Al composition ratio of the low Alcomposition layer 53 is larger than the relaxation rate α of the high Alcomposition layer 52. It was found that the compressive stress occurs inthe low Al composition layer 53 (the low Al composition layer 53 hascompressive strain) in the case where the Al composition ratio of thelow Al composition layer 53 is smaller than the relaxation rate α of thehigh Al composition layer 52.

The lattice spacing along the a-axis when unstrained (the latticeconstant of the a-axis) of the Al_(z)Ga_(1-z)N layer in the case wherethe Al composition ratios z of the Al_(z)Ga_(1-z)N layers are 0.2, 0.35,0.5, and 0.7 are 0.3174 nm, 0.3162 nm, 0.3151 nm, and 0.3135 nm,respectively. In the case of this example as described above, the actuallattice spacing along the a-axis of the AlN which is the high Alcomposition layer 52 is 0.3145 nm. Accordingly, the compressive strainis not formed and the cracks cannot be suppressed simply by forming thelow Al composition layer 53 on the high Al composition layer 52 with anAl composition ratio that is smaller than that of the high Alcomposition layer 52. The compressive strain is formed and the crackscan be suppressed by forming the low Al composition layer 53 with an Alcomposition ratio that reflects the actual lattice spacing along thea-axis of the high Al composition layer 52, i.e., the relaxation rate αof the high Al composition layer 52. In other words, the compressivestrain is formed and the cracks can be suppressed by forming the low Alcomposition layer 53 with an Al composition ratio that is not more thanthe relaxation rate α of the high Al composition layer 52.

The dislocation density will now be described.

FIG. 7 is a graph Illustrating the dislocation density of the nitridesemiconductor.

FIG. 7 illustrates the dislocation density for the four samples recitedabove in which the Al composition ratio z of the Al_(z)Ga_(1-z)N layerwas changed and a sample in which the Al composition ratio z was 0. Thehorizontal axis of FIG. 7 is the Al composition ratio x. The verticalaxis on the left is a screw dislocation density Ds (cm⁻²). The verticalaxis on the right is an edge dislocation density Dt (cm⁻²).

As Illustrated in FIG. 7, the screw dislocation density Ds and the edgedislocation density Dt are high when the Al composition ratio z is 0.The screw dislocation density Ds is low in the range in which the Alcomposition ratio z is 0.2 to 0.5. The screw dislocation density Dsincreases when the Al composition ratio z is 0.7. On the other hand, theedge dislocation density Dt decreases as the Al composition ratio zincreases in the range of 0 to 0.5. The edge dislocation density Dtincreases when the Al composition ratio z is 0.7.

It is considered that the misfit dislocations that occur between the lowAl composition layer 53 and the high Al composition layer 52 aresuppressed as the Al composition ratio z increases in the range of 0 to0.5 because the lattice constant of the low Al composition layer 53approaches the actual lattice spacing of the high Al composition layer52. It is considered that in the case where the Al composition ratio zexceeds 0.5, the lattice constant of the low Al composition layer 53becomes smaller than the actual lattice spacing of the high Alcomposition layer 52; the misfit dislocations increase; and thedislocation density increases.

It was found that a low dislocation density (e.g., the lowestdislocation density) is obtained in the case where the Al compositionratio of the low Al composition layer 53 is about the same as therelaxation rate α of the high Al composition layer 52. The Alcomposition ratio at which the dislocation density is low depends on therelaxation rate α of the high Al composition layer 52. This is becausethe lattice constant of the low Al composition layer 53 at which themisfit dislocations occur changes dependent on the change of the actuallattice spacing of the high Al composition layer 52. When the relaxationrate α Is small, the Al composition ratio at which the dislocationdensity is low decreases. In other words, there is a relativerelationship between the Al composition ratio of the low Al compositionlayer 53 and the relaxation rate α of the high Al composition layer 52;and the Al composition ratio at which the dislocation density is low hasan appropriate range.

FIG. 7 illustrates the results of the case where the relaxation rate αof the high Al composition layer 52 is 0.57. In such a case, thedislocation density abruptly increases when the Al composition ratio ofthe low Al composition layer 53 becomes 0.2 or less. In other words, thedislocation density abruptly increases when the Al composition ratiobecomes 35% or more of the relaxation rate α (in this example, 0.57). Onthe other hand, the tensile strain is formed and the cracks increasewhen the Al composition ratio of the low Al composition layer 53 becomeslarger than the relaxation rate α of the high Al composition layer 52.

Thus, a high-quality nitride semiconductor element having few cracks anddislocations can be provided by the Al composition ratio of the low Alcomposition layer 53 being not more than the relaxation rate α of thehigh Al composition layer 52 for the stacked body 50 having a structurein which the high Al composition layer 52, the low Al composition layer53, and the GaN intermediate layer 51 are stacked in this order.

FIG. 8A to FIG. 8D are schematic views illustrating configurations ofnitride semiconductor elements.

As illustrated in FIG. 8A and FIG. 8B, the c-axis of the nitridesemiconductor layer of the nitride semiconductor element may beperpendicular to the Z-axis direction (a direction perpendicular to themajor surface 60 a of the foundation layer 60 and perpendicular to themajor surface 40 a of the substrate 40). In such a case, the first axisrelating to the lattice spacing may be, for example, an axis parallel tothe (1-100) plane. For example, the first axis may be an axis parallelto the (11-20) plane.

As illustrated in FIG. 8C and FIG. 8D, the c-axis of the nitridesemiconductor layer may be tilted with respect to the Z-axis direction.In such a case, for example, the first axis relating to the latticespacing may be an axis parallel to the (1-101) plane. For example, thefirst axis may be an axis parallel to the (11-22) plane.

These are examples; and any axis parallel to the major surface 60 a ofthe foundation layer 60 (any axis parallel to the major surface 40 a ofthe substrate 40) is applicable as the first axis of the embodiment.

Second Embodiment

The embodiment relates to a nitride semiconductor wafer. For example, atleast a portion of the semiconductor device or a portion that is used toform at least a portion of the semiconductor device is provided in thiswafer. For example, the semiconductor device includes a semiconductorlight emitting element, a semiconductor light receiving element, anelectronic device, etc.

FIG. 9A to FIG. 9D are schematic views illustrating the configuration ofthe nitride semiconductor wafer according to the second embodiment.

FIG. 9A is a schematic cross-sectional view illustrating theconfiguration of the nitride semiconductor wafer 210 according to theembodiment. FIG. 9B is a graph illustrating the Al composition ratio(C_(Al)) of the stacked intermediate layer. FIG. 9C is a graphillustrating the growth temperature GT (the formation temperature) ofthe stacked intermediate layer. FIG. 9D is a graph Illustrating thelattice spacing Ld along the a-axis of the stacked intermediate layer.

As illustrated in FIG. 9A, the nitride semiconductor wafer 210 accordingto the embodiment includes the substrate 40, the foundation layer 60,and the stacked body 50. The stacked body 50 includes the first stackedintermediate layer 50 a; and the nitride semiconductor wafer 210 mayfurther include the functional layer 10. The configurations described inregard to the first embodiment are applicable to the substrate 40, thefoundation layer 60, the stacked body 50, and the functional layer 10.The foundation layer 60 is provided on the major surface 40 a of thesubstrate 40; the stacked body 50 is provided on the foundation layer60; and the functional layer 10 is provided on the stacked body 50. TheAl composition ratio of the low Al composition layer 53 is not more thanthe relaxation rate α of the high Al composition layer 52.

According to the nitride semiconductor wafer 210, a nitridesemiconductor wafer for a high-quality nitride semiconductor elementhaving few cracks and dislocations can be provided.

In the embodiment as well, the GaN layer 11 i (the undoped GaN layer)may be provided on the stacked body 50 (e.g., between the stacked body50 and the functional layer 10).

FIG. 10A to FIG. 10D are schematic views illustrating the configurationof another nitride semiconductor wafer according to the secondembodiment.

FIG. 10A is a schematic cross-sectional view illustrating theconfiguration of the nitride semiconductor wafer 220 according to theembodiment. FIG. 10B to FIG. 10D are graphs illustrating the Alcomposition ratio (C_(Al)), the growth temperature GT, and the latticespacing Ld along the a-axis of the stacked intermediate layer,respectively.

In the nitride semiconductor wafer 220 as illustrated in FIG. 10A, thestacked body 50 includes the first stacked Intermediate layer 50 a andthe second stacked intermediate layer 50 b. The first stackedIntermediate layer 50 a is provided on the foundation layer 60. Forexample, the first stacked Intermediate layer 50 a is provided betweenthe foundation layer 60 and the functional layer 10. The second stackedintermediate layer 50 b is provided between the first stackedintermediate layer 50 a and the functional layer 10.

The first stacked intermediate layer 50 a includes the first GaNIntermediate layer 51 a provided on the foundation layer 60, the firsthigh Al composition layer 52 a provided on the first GaN Intermediatelayer 51 a, and the first low Al composition layer 53 a provided on thefirst high Al composition layer 52 a. The second stacked intermediatelayer 50 b includes the second GaN Intermediate layer 51 b provided onthe first stacked Intermediate layer 50 a, the second high Alcomposition layer 52 b provided on the second GaN Intermediate layer 51b, and the second low Al composition layer 53 b provided on the secondhigh Al composition layer 52 b.

The Al composition ratio of the first low Al composition layer 53 a isnot more than the relaxation rate αa of the first high Al compositionlayer 52 a. The Al composition ratio of the second low Al compositionlayer 53 b is not more than the relaxation rate αb of the second high Alcomposition layer 52 b.

According to the nitride semiconductor wafer 220, a nitridesemiconductor wafer for a high-quality nitride semiconductor elementhaving few cracks and dislocations can be provided.

In this example as well, the GaN layer 11 i (the undoped GaN layer) maybe provided between the stacked body 50 and the functional layer 10.

In the embodiment Illustrated in FIG. 3A or FIG. 10A, the second GaNIntermediate layer 51 b, the second high Al composition layer 52 b, andthe second low Al composition layer 53 b may be considered to be thefirst GaN intermediate layer 51 a, the first high Al composition layer52 a, and the first low Al composition layer 53 a, respectively.

As shown in FIGS. 11F-11J, the δ-doped layer 50 dd of Si may be providedin at least one of the second GaN Intermediate layer 51 b, the secondhigh Al composition layer 52 b and the second low Al composition layer53 b. In addition, the δ-doped layer 50 dd of Si may be provided in atleast one of the second GaN Intermediate layer 51 b, the second high Alcomposition layer 52 b and the second low Al composition layer 53 b,while the δ-doped layer 50 dd of Si is provided in at least one of thefirst GaN Intermediate layer 51 a, the first high Al composition layer52 a and the first low Al composition layer 53 a.

The configuration of the position of the δ-doped layer 50 dd in thesecond GaN Intermediate layer 51 b, the second high Al composition layer52 b and the second low Al composition layer 53 b may be the same as theconfiguration for the case of the first GaN intermediate layer 51 a, thefirst high Al composition layer 52 a and the first low Al compositionlayer 53 a, respectively.

In the embodiment, for example, metal-organic chemical vapor deposition(MOCVD), metal-organic vapor phase epitaxy (MOVPE), molecular beamepitaxy (MBE), hydride vapor phase epitaxy (HVPE), etc., may be used togrow the semiconductor layer.

For example, in the case where MOCVD or MOVPE is used, the following maybe used as the source material when forming each of the semiconductorlayers. For example, TMGa (trimethylgallium) and TEGa (triethylgallium)may be used as the source material of Ga. For example, TMIn(trimethylindium), TEIn (triethylindium), etc., may be used as thesource material of In. For example, TMAI (trimethylaluminum), etc., maybe used as the source material of Al. For example, NH₃ (ammonia), MMHy(monomethylhydrazine), DMHy (dimethylhydrazine), etc., may be used asthe source material of N. SiH₄ (monosilane), Si₂H₆ (disilane), etc., maybe used as the source material of Si.

According to the embodiments, a nitride semiconductor element and anitride semiconductor wafer having few cracks can be provided.

In the specification, “nitride semiconductor” includes all compositionsof semiconductors of the chemical formula B_(x)In_(y)AlGa_(1-x-y-z)N(0<x≦1, 0≦y≦1, 0≦z≦1, and x+y+z≦1) for which the composition ratios x,y, and z are changed within the ranges respectively. “Nitridesemiconductor” further includes group V elements other than N (nitrogen)in the chemical formula recited above, various elements added to controlvarious properties such as the conductivity type and the like, andvarious elements included unintentionally.

In the specification of the application, “perpendicular” and “parallel”refer to not only strictly perpendicular and strictly parallel but alsoinclude, for example, the fluctuation due to manufacturing processes,etc. It is sufficient to be substantially perpendicular andsubstantially parallel.

Hereinabove, exemplary embodiments of the invention are described withreference to specific examples. However, the invention is not limited tothese specific examples. For example, one skilled in the art maysimilarly practice the invention by appropriately selecting specificconfigurations of components included in nitride semiconductor elementsand nitride semiconductor wafers such as substrates, foundation layers,AlN buffer layers, AlGaN foundation layers, GaN foundation layers,stacked Intermediate layers, high Al composition layers, low Alcomposition layers, GaN intermediate layers, functional layers, etc.,from known art; and such practice is included in the scope of theinvention to the extent that similar effects are obtained.

Further, any two or more components of the specific examples may becombined within the extent of technical feasibility and are included inthe scope of the invention to the extent that the purport of theinvention is included.

Moreover, all nitride semiconductor elements and nitride semiconductorwafers practicable by an appropriate design modification by one skilledin the art based on the nitride semiconductor element and nitridesemiconductor wafer described above as embodiments of the invention alsoare within the scope of the invention to the extent that the spirit ofthe invention is included.

Various other variations and modifications can be conceived by thoseskilled in the art within the spirit of the invention, and it isunderstood that such variations and modifications are also encompassedwithin the scope of the invention.

While certain embodiments have been described, these embodiments havebeen presented by way of example only, and are not intended to limit thescope of the inventions. Indeed, the novel embodiments described hereinmay be embodied in a variety of other forms; furthermore, variousomissions, substitutions and changes in the form of the embodimentsdescribed herein may be made without departing from the spirit of theinventions. The accompanying claims and their equivalents are intendedto cover such forms or modifications as would fall within the scope andspirit of the invention.

1-20. (canceled) 21: A nitride semiconductor element, comprising: astacked body, wherein the stacked body comprises: a GaN intermediatelayer; a low Al composition layer comprising a nitride semiconductorhaving a first Al composition ratio; a high Al composition layercomprising a nitride semiconductor having a second Al composition ratio,the high Al composition layer being present between the GaN intermediatelayer and the low Al composition layer, the second Al composition ratiobeing higher than the first Al composition ratio; and a firstSi-containing region provided between the GaN intermediate layer and thehigh Al composition layer, the first Si-containing region comprisingSiN. 22: The nitride semiconductor element according to claim 21,wherein a Si concentration of the first Si-containing region is not lessthan 7.0×10¹⁹/cubic centimeter and not more than 5.0×10²⁰/cubiccentimeter. 23: The nitride semiconductor element according to claim 21,wherein a thickness of the first Si-containing region is not less than0.3 nanometers and less than 200 nanometers. 24: The nitridesemiconductor element according to claim 21, wherein the firstSi-containing region comprises a delta-doped layer of silicon. 25: Thenitride semiconductor element according to claim 21, wherein the stackedbody further comprises a second Si-containing region provided betweenthe low Al composition layer and the high Al composition layer, thesecond Si-containing region comprising SiN. 26: The nitridesemiconductor element according to claim 25, wherein a Si concentrationof the second Si-containing region is not less than 7.0×10¹⁹/cubiccentimeter and not more than 5.0×10²⁰/cubic centimeter. 27: The nitridesemiconductor element according to claim 25, wherein a thickness of thesecond Si-containing region is not less than 0.3 nanometers and lessthan 200 nanometers. 28: The nitride semiconductor element according toclaim 25, wherein the second Si-containing region comprises adelta-doped layer of silicon. 29: The nitride semiconductor elementaccording to claim 21, wherein the high Al composition layer comprisesat least one selected from Al_(x1)Ga_(1-x1)N (0<x1≦1) and aluminumnitride. 30: The nitride semiconductor element according to claim 29,wherein the low Al composition layer comprises Al_(y1)Ga_(1-y1)N (0<y1<1and y1<x1). 31: The nitride semiconductor element according to claim 21,further comprising: a substrate; and a foundation layer, wherein thefoundation layer is provided between the substrate and the stacked body.32: The nitride semiconductor element according to claim 31, wherein thesubstrate comprises silicon. 33: The nitride semiconductor elementaccording to claim 31, wherein the foundation layer comprises aluminumnitride. 34: A nitride semiconductor element, comprising a functionallayer formed on a stacked body, wherein the stacked body comprises: aGaN intermediate layer; a low Al composition layer comprising a nitridesemiconductor having a first Al composition ratio, the low Alcomposition layer being provided between the GaN intermediate layer andthe functional layer; a high Al composition layer comprising a nitridesemiconductor having a second Al composition ratio, the high Alcomposition layer being provided between the GaN intermediate layer andthe low Al composition layer, the second Al composition ratio beinghigher than the first Al composition ratio; and a first Si-containingregion provided between the GaN intermediate layer and the high Alcomposition layer, the first Si-containing region comprising SiN. 35:The nitride semiconductor element according to claim 34, wherein atleast a portion of the stacked body is removed after the functionallayer is formed on the stacked body. 36: A nitride semiconductorelement, comprising: a stacked body, wherein the stacked body comprises:a GaN intermediate layer; a low Al composition layer comprising anitride semiconductor having a first Al composition ratio, the low Alcomposition layer being provided between the GaN intermediate layer andthe functional layer; a high Al composition layer comprising a nitridesemiconductor having a second Al composition ratio, the high Alcomposition layer being provided between the GaN intermediate layer andthe low Al composition layer, the second Al composition ratio beinghigher than the first Al composition ratio; and a second Si-containingregion provided between the low Al composition layer and the high Alcomposition layer, the second Si-containing region comprising SiN. 37: Anitride semiconductor wafer, comprising: a substrate; and a stacked bodyprovided on the substrate, wherein the stacked body comprises: a GaNintermediate layer; a low Al composition layer comprising a nitridesemiconductor having a first Al composition ratio, the low Alcomposition layer being provided between the GaN intermediate layer andthe functional layer; a high Al composition layer comprising a nitridesemiconductor having a second Al composition ratio, the high Alcomposition layer being provided between the GaN intermediate layer andthe low Al composition layer, the second Al composition ratio beinghigher than the first Al composition ratio; and a first Si-containingregion provided between the GaN intermediate layer and the high Alcomposition layer, the first Si-containing region comprising SiN. 38:The nitride semiconductor wafer according to claim 37, wherein a Siconcentration of the first Si-containing region is not less than7.0×10¹⁹/cubic centimeter and not more than 5.0×10²⁰/cubic centimeter,and a thickness of the first Si-containing region is not less than 0.3nanometers and less than 200 nanometers.